Techniques for processing devices

ABSTRACT

Representative techniques provide process steps for forming a microelectronic assembly, including preparing microelectronic components such as dies, wafers, substrates, and the like, for bonding. One or more surfaces of the microelectronic components are formed and prepared as bonding surfaces. The microelectronic components are stacked and bonded without adhesive at the prepared bonding surfaces.

PRIORITY CLAIM AND CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit under 35 U.S.C. § 119(e)(1) of U.S.Provisional Application No. 62/631,216, filed Feb. 15, 2018, which ishereby incorporated by reference in its entirety.

FIELD

The following description relates to integrated circuits (“ICs”). Moreparticularly, the following description relates to manufacturing IC diesand wafers.

BACKGROUND

Microelectronic elements often comprise a thin slab of a semiconductormaterial, such as silicon or gallium arsenide, commonly called asemiconductor wafer. A wafer can be formed to include multipleintegrated chips or dies on a surface of the wafer and/or partlyembedded within the wafer. Dies that are separated from a wafer arecommonly provided as individual, prepackaged units. In some packagedesigns, the die is mounted to a substrate or a chip carrier, which isin turn mounted on a circuit panel, such as a printed circuit board(PCB). For example, many dies are provided in packages suitable forsurface mounting.

Packaged semiconductor dies can also be provided in “stacked”arrangements, wherein one package is provided, for example, on a circuitboard or other carrier, and another package is mounted on top of thefirst package. These arrangements can allow a number of different diesto be mounted within a single footprint on a circuit board and canfurther facilitate high-speed operation by providing a shortinterconnection between the packages. Often, this interconnect distancecan be only slightly larger than the thickness of the die itself. Forinterconnection to be achieved within a stack of die packages,interconnection structures for mechanical and electrical connection maybe provided on both sides (e.g., faces) of each die package (except forthe topmost package).

Additionally, dies or wafers may be stacked in a three-dimensionalarrangement as part of various microelectronic packaging schemes. Thiscan include stacking a layer of one or more dies, devices, and/or waferson a larger base die, device, wafer, substrate, or the like, stackingmultiple dies or wafers in a vertical or horizontal arrangement, andvarious combinations of both. Dies or wafers may be bonded in a stackedarrangement using various bonding techniques, including directdielectric bonding, non-adhesive techniques, such as ZiBond® or a hybridbonding technique, such as DBI®, both available from Invensas BondingTechnologies, Inc. (formerly Ziptronix, Inc.), an Xperi company (see forexample, U.S. Pat. Nos. 6,864,585 and 7,485,968, which are incorporatedherein in their entirety).

There can be a variety of challenges to implementing stacked die andwafer arrangements. When bonding stacked dies using a direct bonding orhybrid bonding technique, it is usually desirable that the surfaces ofthe dies to be bonded be extremely flat, smooth, and clean. Forinstance, in general, the surfaces should have a very low variance insurface topology and have a low level of impurities, particles, or otherresidue. The removal of particles or residues can improve thecleanliness and flatness of the surfaces and the reliability of the bondbetween the layers, however, the removal of particles and residue cansometimes be problematic.

BRIEF DESCRIPTION OF THE DRAWINGS

The detailed description is set forth with reference to the accompanyingfigures. In the figures, the left-most digit(s) of a reference numberidentifies the figure in which the reference number first appears. Theuse of the same reference numbers in different figures indicates similaror identical items.

For this discussion, the devices and systems illustrated in the figuresare shown as having a multiplicity of components. Variousimplementations of devices and/or systems, as described herein, mayinclude fewer components and remain within the scope of the disclosure.Alternatively, other implementations of devices and/or systems mayinclude additional components, or various combinations of the describedcomponents, and remain within the scope of the disclosure.

FIG. 1A shows an example profile view of a substrate and two dies priorto bonding the dies to the substrate.

FIG. 1B shows an example profile view of the substrate and the two diesafter bonding the dies to the substrate.

FIG. 1C shows an example profile view of the substrate and bondingmultiple dies to the substrate.

FIG. 2 is a flow diagram illustrating an example process for preparing asubstrate for bonding, according to an embodiment.

FIG. 3 is a flow diagram illustrating an example process for preparingdies for bonding, according to an embodiment.

FIG. 4 is a flow diagram illustrating another example process forpreparing dies for bonding, according to an embodiment.

FIG. 5 is a flow diagram illustrating another example process forpreparing dies for bonding, according to an embodiment.

FIG. 6A is a flow diagram illustrating an example process for preparinga substrate for bonding, according to an embodiment.

FIG. 6B is a flow diagram illustrating an example process for preparingdies for bonding, according to an embodiment.

FIG. 7A is a flow diagram illustrating another example process forpreparing a substrate for bonding, according to an embodiment.

FIG. 7B is a flow diagram illustrating another example process forpreparing dies for bonding, according to an embodiment.

SUMMARY

Representative techniques and devices are disclosed, including processsteps for forming a novel microelectronic assembly. Processes includepreparing microelectronic components such as dies, wafers, substrates,and the like, for bonding. In various implementations, one or moresurfaces of the microelectronic components are formed and prepared asbonding surfaces. The microelectronic components may be stacked andbonded without adhesive at the prepared bonding surfaces.

In various implementations, a method for forming a microelectronicassembly includes preparing a bonding surface of a first substrate,including: planarizing the bonding surface of the first substrate tohave a first predetermined minimal surface variance and plasmaactivating the bonding surface of the first substrate. The methodfurther includes preparing a first bonding surface of a secondsubstrate, including planarizing the first bonding surface of the secondsubstrate to have a second predetermined minimal surface variance.

In an implementation, the method includes mounting the second substrateto a dicing sheet or dicing tape held by a dicing frame and singulatingthe second substrate into a plurality of dies while the second substrateis mounted to the dicing sheet or dicing tape. Each die of the pluralityof dies has a first bonding surface comprised of a portion of the firstbonding surface of the second substrate.

The method may include processing the plurality of dies while theplurality of dies is mounted to the dicing sheet or dicing tape,selecting a die of the plurality of dies using a pick and place tool,and directly bonding the first bonding surface of the die to the bondingsurface of the first substrate without adhesive, and without activatingthe first bonding surface of the die.

In additional implementations, techniques and methods include preparinga back side of the second substrate or the plurality of dies (or thesingle die) for bonding an additional die (or multiple additional dies)to the bonded die.

In some implementations, unique sets of processing steps are used toclean the bonding surfaces of the first and/or second substrates, or thedie. For example, in some embodiments, one or more unique formulary maybe used for cleaning. In other embodiments, a megasonic transducer,and/or a mechanical brush may be used to assist in cleaning.

In an alternate implementation, the bonding surfaces of one or more ofthe substrates or dies may be coated with one or more protectivecoatings during processing. For example, in one embodiment, individualprotective coatings may render the bonding surfaces hydrophobic orhydrophilic. The coatings may prevent contamination to bonding surfaces,protect against damage to the bonding surfaces during processing, or thelike. One or more of the coatings maybe removed during other processsteps. Additionally, one or more of the processing tools that handlecomponents may be coated or formed to be hydrophobic as a technique forpreventing contamination of clean components during handling.

Various implementations and arrangements are discussed with reference toelectrical and electronics components and varied carriers. Whilespecific components (i.e., dies, wafers, integrated circuit (IC) chipdies, substrates, etc.) are mentioned, this is not intended to belimiting, and is for ease of discussion and illustrative convenience.The techniques and devices discussed with reference to a wafer, die,substrate, or the like, are applicable to any type or number ofelectrical components, circuits (e.g., integrated circuits (IC), mixedcircuits, ASICS, memory devices, processors, etc.), groups ofcomponents, packaged components, structures (e.g., wafers, panels,boards, PCBs, etc.), and the like, that may be coupled to interface witheach other, with external circuits, systems, carriers, and the like.Each of these different components, circuits, groups, packages,structures, and the like, can be generically referred to as a“microelectronic component.” For simplicity, unless otherwise specified,components being bonded to another component will be referred to hereinas a “die,” and the other component that the die is bonded to will bereferred to herein as a “substrate.”

Implementations are explained in more detail below using a plurality ofexamples. Although various implementations and examples are discussedhere and below, further implementations and examples may be possible bycombining the features and elements of individual implementations andexamples.

DETAILED DESCRIPTION Overview

Referring to FIGS. 1A-1C, in various examples, techniques for processingdevices to be stacked and directly or intimately bonded are disclosed.The disclosure describes example processes that include bonding one ormore dies 102 to a substrate 104. However, the processes can also beused to bond wafers to wafers, dies to dies, dies to wafers, and soforth. The diagrams in the accompanying figures show some processes forforming and preparing a substrate 104 (and particularly a bondingsurface 108 of a substrate 104) for bonding, and other processes forforming and preparing the one or more dies 102 (and particularly thebonding surfaces 106 and 110 of the dies 102) for bonding to thesubstrate 104 and to other dies 102. At the end of these processes, theone or more dies 102 are bonded (directly bonded, for example, withoutan adhesive) to the substrate 104 or to another die 102. The bonding isa spontaneous process that takes place at ambient conditions when thetwo surfaces (106 and 108 and/or 106 and 110) are brought together.

Profile views showing example dies 102 and an example substrate 104 areillustrated at FIGS. 1A-1C. FIG. 1A illustrates dies 102 and thesubstrate 104 prior to bonding, and FIG. 1B shows the dies 102 and thesubstrate 104 after bonding. FIG. 1C shows bonding multiple dies 102 tothe substrate 104, for instance, by stacking and bonding dies 102B todies 102A, after bonding the dies 102A to the substrate 104. Additionaldies 102(C-N) may be stacked and bonded to the dies 102B as desired inlike manner.

In various embodiments, an example die 102 or an example substrate 104can include one or more conductive traces or interconnects (not shown)surrounded by an insulating material (oxide, for example) on the bondingsurface (106, 110) of the die 102 or the bonding surface 108 of thesubstrate 104. For example, a die 102 or a substrate 104 can includemultiple conductive and insulating layers (not shown) to determine thefunction (e.g., memory, processing, programmable logic, etc.) of the die102 or the substrate 104. Exposed interconnects on the bonding surface(106, 108, or 110) can be electrically continuous with the variousconductive layers, and provide an interface for the die 102 or thesubstrate 104.

When bonding the dies 102 to the substrate 104 (or bonding dies to dies,dies to wafers, wafers to wafers, etc.) using a direct bonding or hybridbonding technique, it is usually desirable that the surfaces of the dies102 and substrate 104 to be bonded be extremely flat and smooth. Forinstance, in general, the bonding surfaces (106 and 108, and 106 and110) should have a very low variance in surface topology (i.e.,nanometer scale variance, for instance below 2 nm and preferably below0.5 nm), so that the bonding surfaces (106 and 108, and 106 and 110) canbe closely mated to form a lasting bond. It is also usually desirablethat the bonding surfaces (106 and 108, and 106 and 110) be clean andhave a low level of impurities, particles, or other residue that arelarge enough in size to cause bonding voids that can cause electricalcontinuity failures or other bonding defects.

For instance, particles and residues remaining from processing steps canresult in voids at the bonding interfaces between the stacked dies 102and the substrate 104. If the voids are substantially smaller than themetallic electrical interconnect size, they may be acceptable. However,particles that cause bonding defects in sizes that are close to orexceed the electrical interconnect size often cannot be tolerated. Also,temporarily bonding dies, wafers, and substrates for processing orhandling (e.g., using polymeric or inorganic layers, etc. for temporarybonds) can also be problematic, since the temporary carriers andsubstrates can leave behind bonding residue when removed, which canpersist even after cleaning.

Residue from temporary adhesive layers, which can be comprised of hightemperature polymers and the like, can be discontinuous with varyingthicknesses on the die surfaces 106 and 110 or the substrate surface 108(e.g., thickness may range from 10 nm to 50 um). Plasma cleaning, suchas ashing with oxygen plasma, can be used to remove thin residue, buteven long oxygen plasma ashing steps (e.g., over 40 minutes) may beineffective to remove the thickest residues. Additionally, long oxygenashing processes can tend to oxidize the conductive features on thebonding surfaces (106, 110, and/or 108) and reduce the planarity of thecleaned surfaces.

For example, a bonding surface (106, 110, and/or 108) may includerecessed conductive features, where the nominal conductive layer recessis about 1 nm to 20 nm below the surface of an adjacent dielectric layeror surface. After long oxygen ashing of the wafer or die surface, theformerly recessed conductive features may change substantially tointerfere with bonding or with formation of metal interconnects afterbonding. For instance, the formerly recessed conductive features may nowprotrude over the surface of adjacent dielectric regions. The protrusionof the conductive features may vary from 3 nm to 30 nm in some cases,depending on the ashing process parameters. These conductive asperitiesover the bonding surface (106, 110, and/or 108) can hinder or preventintimate mating of bonding layers or surfaces.

In such cases, a high temperature (e.g., over 50° C.) wet process issometimes used to remove thick residue; however, such a process may notbe compatible with other die 102 or substrate 104 layers or materials.For instance, a high temperature wet process can degrade the polishedmetal layers of the die 102 or substrate 104, reducing device yield.Further, the high temperatures may not be compatible with some die 102or substrate 104 processing components (e.g., plastic dicing tapes, griprings, etc.).

In some conventional wafer cleaning processes, wet chemical cleaningsolutions, including aggressive cleaning solutions may be effective inremoving organic residues from surfaces of wafers. For example, cleaningwafers with mixtures of hydrogen peroxide in a solution of ammoniumhydroxides, sulfuric acid, or hydrochloric acids, and their variouscombinations, at temperatures between 50 and 80° C. can produce pristinewafer surfaces. However, such aggressive cleaning steps would generallybe impractical for cleaning the bonding surfaces 106 and 110 of the dies102, since these aggressive wet chemistries will not only clean thepolymeric residues and particles, they may also dissolve large portionsof the practical conductive features, which are often comprised ofmaterials such as silver, copper, nickel, and their various alloys.

Representative Processes

FIGS. 2-7B illustrate representative processes 200-750 for forming andpreparing microelectronic components (such as dies 102 and substrates104, for example) for bonding, such as for direct bonding withoutadhesive. The processes 200-750 include providing a bonding surface(such as bonding surfaces 106, 110, and 108, for example) on themicroelectronic components, or two bonding surfaces in some examples(such as bonding surfaces 106 and 110 on a single die 102), planarizingthe bonding surfaces, cleaning and activating (in some examples) thebonding surfaces, and the like.

The order in which the processes 200-750 are described is not intendedto be construed as limiting, and any number of the described processblocks in any of the processes 200-750 can be combined in any order toimplement the processes, or alternate processes. Additionally,individual blocks may be deleted from any of the processes withoutdeparting from the spirit and scope of the subject matter describedherein. Furthermore, the processes 200-750 can be implemented in anysuitable hardware, software, firmware, or a combination thereof, withoutdeparting from the scope of the subject matter described herein.

In alternate implementations, other techniques may be included in theprocesses 200-750 in various combinations and remain within the scope ofthe disclosure.

Example Technique for Forming and Preparing the Substrate

An example substrate 104 forming process 200 is illustrated at FIG. 2.In various embodiments, the substrate 104 may be comprised of silicon,germanium, a dielectric surface, direct or indirect gap semiconductormaterials or layers or another suitable material. At block 202, theprocess includes fabricating the substrate 104. Briefly, forming thesubstrate 104 may include fabrication of devices in the substrate 104such as the front end of the line (FEOL), multilayer backend of the line(BEOL) and other structures of interest, cleaning of the surface of thesubstrate 104, and so forth.

At block 204, the process includes forming a direct bonding layer on afirst (front) surface 108 of the substrate 104. The materials for thebonding layer may be deposited or formed on the first surface 108, andmay be comprised of an inorganic dielectric material layer such asoxide, nitride, oxynitride, oxycarbide, carbides, nitrocarbides,diamond, diamond like materials, glasses, ceramics, glass-ceramics, andthe like, or a combination of an inorganic dielectric layer and one ormore metal features. In some embodiments, the metal features can berecessed from the dielectric surface slightly, e.g., 1 nm to 20 nm belowthe surface of the dielectric. In some embodiments, forming the directbonding layer can be incorporated into the wafer fabrication process(such as block 202, for example) as the last metal layer formed. In oneembodiment, the planar surface of the last metallized dielectric layermay comprise the bonding surface 108 and additional metallizeddielectric coating to form the bonding layer 108 may not be necessary.

Forming the direct bonding layer includes finishing the first surface108 (i.e., bonding surface) to meet dielectric roughness specificationsand metallic layer (e.g., copper, etc.) recess specifications to preparethe surface 108 for direct bonding. In other words, the bonding surface108 is formed to be as flat and smooth as possible, with very minimalsurface topology variance. Various conventional processes, such aschemical mechanical polishing (CMP) may be used to achieve the lowsurface roughness. The metallic layer may be configured to provide anelectrical and/or thermal path or may instead be configured to balanceout the metallization, through the use of so-called dummy pads, traces,patterns or the like.

At block 206, the bonding surface 108 of the substrate 104 is cleanedand/or activated in preparation for direct bonding, as described below.

Example Technique for Forming and Preparing the Dies

FIG. 3 illustrates an example process 300 for forming single-sided ordouble sided dies 102 from a wafer (which also may be referred to as a“second substrate,” and which may be comprised of similar or samematerials and formed with similar or same processes and techniques asdescribed above with reference to the substrate 104). In animplementation, the process steps 302 and 304 of fabricating the waferand forming the (front) bonding surface 106, with the surface topologyrequirements of the bonding surface, are essentially the same asdescribed above with reference to the substrate 104, at blocks 202 and204 of process 200.

At block 306, after planarizing the bonding surface 106 of the wafer(e.g., by a CMP process, for example) to achieve a desired topology, aprotective coating (such as a resist or other suitable material) isapplied to the bonding surface of the wafer to protect the bondingsurface 106 against contamination, to protect the exposed metallic layeragainst corrosion, and to protect the bonding surface 106 duringsingulation operations, which can tend to generate debris on the frontand side surfaces of the dies 102.

Additionally or alternatively, the back surface of the wafer may beprocessed (e.g. through-die conductor reveal, planarization, etc.),forming a back bonding surface 110 on the dies 102, which may also becoated with a protective coating, or the like.

In some embodiments, more than one type of protective layer may beapplied to the wafer surface. For example, the first protective layermay comprise a hydrophobic protective layer and overlying thehydrophobic layer may be a hydrophilic protective layer. In the example,the underlying hydrophobic layer allows for the use of aggressiveetching chemicals during cleaning of the side surfaces of the dies 102and also increases the shelf life of the prepared surfaces 106 and 110.In some cases, the overlying hydrophilic layer receives or isimpregnated with particles, debris, dicing tape, adhesive, etc.generated during the dicing process. The hydrophilic layer, along withthe particles and debris, is removed. The hydrophobic layer may betemporarily maintained intact to protect the surface 106 (and/or 110)during subsequent processing or storage.

In some instances, as shown at block 308, the wafer is thinned from thenon-protected side to achieve a desired thickness. At block 310 thewafer is mounted to a dicing tape on a frame and singulated (block 312)to form the dies 102. The wafer may be singulated into a quantity ofdies 102 using a saw, a laser (e.g. stealth laser), a plasma etchingprocess, or other suitable technique. In various examples, the dies 102are singulated while the wafer is mounted to a dicing sheet or dicinglayer or the dicing tape (or the like), held by the frame.

After dicing, the diced wafer on the dicing tape can be expanded andmounted into a grip ring if desired. In various examples, the frame orgrip ring may be comprised of plastic parts, metal parts, combinationsof the same, and the like. The dicing tape can be any type of dicingtape commonly used in the industry. One example dicing tape includes aUV release tape. In some examples, the dies 102 may be shipped while onthe grip ring or dicing frame.

Forming and Preparing the Dies—Double-Sided Die Examples

FIG. 4 illustrates an example process 400 for forming double-sided dies102, where both surfaces (106 and 110) of a die 102 will be bonded tosubstrates 104 or to other dies 102, such as with multiple die-to-die ordie-to-wafer applications (as shown in FIG. 1C, for example). At block402, the process includes fabricating the wafer for the dies 102, whichmay include fabricating through-silicon-vias (TSVs) at desired locationsthroughout the dies 102 for future through-device connections. Invarious embodiments, the wafer may be fabricated from silicon,germanium, or another suitable material.

At block 404, the process includes forming a direct bonding layer on afirst (front) surface 106 of the dies 102. The materials for the bondinglayer may be deposited or formed on the first surface 106 and may becomprised of an inorganic dielectric material layer or a combination ofan inorganic dielectric layer and one or more metal features, asdescribed with reference to FIG. 2. Similarly, as discussed earlier, theplanar surface of the last metallized dielectric layer may comprise thebonding surface 106 and additional metallized dielectric coating to formthe bonding layer 106 may not be necessary.

Forming the direct bonding layer includes finishing the first surface106 (i.e., bonding surface) to meet dielectric roughness specificationsand metallic layer (e.g., copper, etc.) recess specifications to preparethe surface 106 for direct bonding. In other words, the bonding surface106 is formed to be as flat and smooth as possible, with very minimalsurface topology variance. Various conventional processes, such aschemical mechanical polishing (CMP) may be used to achieve the lowsurface roughness.

In an implementation, at block 406 the prepared front surface of thewafer to be singulated into dies 102 is directly bonded to a supportwafer (e.g., a “silicon carrier” in one example, but carriers of othermaterials may also be used), for handling the wafer during fabricationof the second (back) bonding surface 110. In one example, the firstsurface of the wafer is bonded to a silicon wafer using a Zibond directbonding technique, as described above.

In various embodiments, directly bonding the silicon carrier to thewafer has multiple advantages. For instance, this technique removes anytemperature limitations for back-side wafer processing. Further, thesilicon carrier can have a similar or closely matched coefficient ofthermal expansion (CTE), reducing or eliminating warping of the waferduring processing. This can improve the success and reliability ofdirect bonds with the wafer. Also, directly bonding the silicon carrierenables an ultra-thin wafer to be directly bonded to other dies, wafers,substrates, etc., where it would not be possible otherwise.

At block 408, prior to forming and polishing the second bonding surface110, the silicon substrate may be thinned, and the TSVs exposed. Thesecond bonding surface 110 of the dies 102, on the back side of thewafer, is formed and finished (as described above) to meet maximumdielectric roughness specifications and metallic layer (e.g., copper,etc.) recess specifications with minimal surface topology variance.Moreover, a protective coating may be applied to the polished secondbonding surface 110.

At block 410, the second surface 110 of the wafer may then be bonded toa temporary support wafer using an adhesive, to handle the wafer forfront-side 106 processing. At block 412, the silicon support wafer isremoved from the first surface 106 of the wafer, using one or moretechniques, including etching, dissolving, grinding, or the like. Theremoval process is performed to ensure that the first surface 106retains the dielectric layer and any conductive layers for future director hybrid bonding after the support wafer has been removed.

At block 414, one or more protective coatings may be applied to theexposed first surface 106. In some embodiments, more than one type ofprotective layer may be applied to the first surface 106. For example, afirst protective layer may comprise a hydrophobic protective layer andoverlaying the hydrophobic layer may be a hydrophilic protective layer(as described above).

At block 416, the temporary carrier and temporary bonding adhesive areremoved from the back surface 110 of the wafer. At block 418, thefinished double-sided wafer is then mounted to a dicing tape on a frameand singulated to form a quantity of double-sided dies 102 (block 420).Optionally, the diced wafer may be transferred to a grip ring (block422) in preparation for bonding (block 424).

Referring to FIG. 5, in an alternate embodiment, illustrated by theprocess 500, the silicon carrier is not bonded to the wafer using adirect bonding technique. Instead, after fabricating (block 502) andforming/finishing the first bonding surface (block 504) as describedabove with reference to FIG. 4, the first surface 106 is bonded to asupporting carrier using a temporary bonding adhesive (block 506) forfabrication of the second (back side) 110 bonding surface.

At block 508, the wafer may be thinned as desired, exposing the TSVs.The second bonding layer 110 is formed and finished as described above.At block 510, one or more protective coatings may be applied to theprepared bonding surface(s) (106 and/or 110) prior to dicing. Afterforming and finishing the second bonding surface 110, the temporarycarrier and temporary adhesive material is removed (block 512). Thefinished double-sided wafer is then mounted to a dicing tape or dicinglayer on a frame (block 514), for example, and singulated (block 516) toform a quantity of double-sided dies 102. Optionally, the diced wafermay be transferred to a grip ring (block 518) in preparation for bonding(block 520).

Cleaning and Preparing the Substrate

The illustrations of FIGS. 6A and 6B show example processes 600 and 650respectively, describing cleaning and preparing the substrate 104(process 600) and the dies 102 (process 650) for bonding. Again,although the illustrations (and the associated descriptions) describebonding dies 102 to a substrate 104, the process can be used to bondwafers to wafers, dies to dies, dies to wafers, and so forth. Moreover,the substrate 104 described in these processes can also refer to theback side 110 of a die 102 already mounted to another die 102 or to alarger substrate, such as a wafer for example.

Referring to FIG. 6A, a process 600 is shown for preparing the bondingsurface 108 of the substrate 104. In some embodiments, the process 600follows or is summarized within block 206 of the process 200 illustratedat FIG. 2. For instance, at block 602, the bonding surface 108 of thesubstrate 104 is cleaned, for example, with a wet process with solvent.Block 604 follows, including a dry clean process with an oxygen plasma(commonly referred to as ashing) or other plasma chemistry, to removeany inorganic and organic contaminants. The plasma may be providedthrough an atmospheric, downstream, or reactive ion etching (RIE)process, for instance.

The substrate 104 may then be re-cleaned, including scrubbed withdeionized or mildly alkaline water or other suitable solution to removeany particles from the ashing process. At block 606, the bonding surface108 is activated using a nitrogen plasma process, for example, toprepare the substrate surface 108 for bonding. In various examples, theactivation process is known to improve bond strength of the bonded die102 to the substrate 104. Similarly, the activation step may be providedthrough an atmospheric, downstream, or reactive ion etching (RIE)process, for example. The substrate 104 may also be rinsed withdeionized water to remove particles from the activation process, asshown at block 608.

Cleaning and Preparing the Dies

Referring to FIG. 6B, a process 650 is shown for preparing the firstbonding surface 106 of the die(s) 102. In some embodiments, the process650 follows or is summarized within blocks 316, 424, or 520 from FIGS.3, 4, and 5, respectively, or the like. For instance, at block 652,preparing the first bonding surface 106 of the dies 102 includesremoving the protective layer from the bonding surface 106. At blocks654 and 656, the bonding surface(s) 106 are cleaned. For example, one ormore alternate or optional process stages, including a wet cleaning witha solvent, a dry cleaning with an oxygen plasma (commonly referred to asashing) or the use of other plasma chemistry, or the like, may be usedto remove any inorganic and organic contaminants. The dry cleaning maybe performed by a plasma process such as an atmospheric, downstream, orRIE process.

In some cases, the protective layer(s) may be cleaned from the dies 102while the dies 102 are held on the dicing tape by the frame or grip ring(via a wet process, for example), which also cleans debris resultingfrom the dicing process. When a UV release tape is used, UV exposure toreduce adhesion strength between the die 102 and the tape to release thedie 102 can be performed either before the die cleaning process, orafter the die cleaning process, if desired.

In various embodiments, the dies 102 are cleaned with chemicals (e.g.,solvents, etchants, water, etc.) that can remove the protective coatingswithout corroding the metallic layer (e.g., copper) underneath. Forexample, the cleaning chemicals may include mixtures of hydrogenperoxide in a solution of ammonium hydroxides, or acids. Metalpassivating compounds (e.g. triazole moieties) may be used to inhibitmetal etching of conductive features such as silver, copper, nickel, andtheir various alloys. These chemicals and passivating compounds may becleaned or rinsed from the metal portions of the bonding surface 106.

During the dicing process, particles of the wafer, for example siliconparticles, can be embedded in the dicing tape by the mechanical actionof the dicing wheel or wire. These embedded particles may contaminateedges and surfaces of the cleaned dies 102, including the bondingsurfaces 106, during pick and place direct die transfer from the dicingtape to the bonding layer. Accordingly, it is very important to reduceor eliminate embedded particles from the cleaned bonding surfaces 106.In one example, the dies 102 may be re-mounted onto a sheet of freshdicing tape of the same or similar material or another suitablematerial, discarding the original dicing tape. This can be done toremove the embedded contaminants or to enable the use of chemicals thatare not compatible with the original dicing tape.

Alternatively, the contaminants trapped in the dicing tape can beremoved with more vigorous cleaning of the dies 102, including megasonicshaking, mechanical brushing, and/or a high pressure wash. The vigorouscleaning, including megasonic shaking, mechanical brushing, and highpressure wash also cleans edges of the dies 102 to remove particles andorganic contaminants at the edges.

The dies 102 may be dry cleaned with a plasma process such as oxygenplasma and re-cleaned with a wet process while on the dicing tape heldby the frame or grip ring, to remove any remaining residue of theprotective layer(s) or additional contaminants resulting from someprocess steps, and to improve the bonding surface 106. However, in somecases, the dicing tape can react with the plasma process and canpotentially cause re-contamination of the bonding surface 106 by itsreaction products.

In some cases, shielding the exposed tape beyond the die 102 andminimizing the exposed tape between the dies 102 (e.g., using a narrowcutting blade and not stretching the tape) and shortening the plasmaprocess duration can reduce the contamination. In some cases, anoxygen-based reactive ion etching (RIE) plasma process is preferred toshorten the ashing process. In various implementations, a short ashingprocess in a RIE plasma chamber can cause less surface re-contaminationthan a long ashing process in a less powerful plasma chamber. There-cleaning can comprise rinsing the dies 102 with deionized water,which can be combined with mechanical brushing, megasonic shaking,and/or a high pressure wash.

At block 658, the dies 102 may be activated. A nitrogen-based RIEprocess or downstream plasma method or other plasma cleaning stepsincluding atomic layer cleaning methods can be used to activate the diesurface 106 and/or clean any remaining residue or undesirable materialsfrom the surfaces of interest. At block 660, as an option, the dies 102may be rinsed with deionized water for a final cleaning.

At the end of the die 102 forming and preparation process, the dies 102may be bonded to the prepared bonding surface 108 of the substrate 104(as illustrated in FIG. 1B). For a multiple-die 102 stacking arrangementas shown in FIG. 1C, after placing each die 102A on the substrate 104(or previous die 102N), the second bonding surface 110 (backside) of theplaced die 102A may be planarized, cleaned, and activated (in someembodiments as needed and as discussed above) before direct bonding abonding surface 106 of a next die 102B to the backside surface 110 ofthe previous die 102A. Any added die 102N may have a prepared bondingsurface 106 and/or 110, one or both of which may or may not beactivated.

In some cases, the nitrogen-based plasma process can be very reactivewith the dicing tape and cause bonding surface contamination, which cannegate the benefits of plasma activation. Plasma processes (as used inashing and activation) can also react with the metal device layer on adie 102 or substrate 104 and change the recess of the metal layer fromthe dielectric surface. Some of the compounds formed as a result of theinteraction of the nitrogen plasma with the polymeric dicing tape mayadsorb on the bonding surfaces of interest and contaminate the bondingsurfaces (106, 108, and in some embodiments 110).

Consequently, in some embodiments, at least one of the bonding surfaces106 and/or 110 may not be activated. In some embodiments, a more gentleoxygen or nitrogen plasma of various combinations is preferred to modifythe surfaces (106, 108, and in some embodiments 110) when dicing tapesare present or in close proximity. Similarly, the activated surface(106, 108, and in some embodiments 110) may be further cleaned. Thecleaning step may comprise a rinse with deionized water, which may becombined with mechanical brushing, megasonic cleaning, and/or a highpressure wash.

In various embodiments, some die process steps may be modified oreliminated, in comparison to the process steps described herein.

Cleaning and Activating Dies on the Pick and Place Tool

In an embodiment, a die 102 is selected by pick and place tools(generally comprising a pick tool and a bonding or placement tool),which may take the die 102 through a cleaning process on the way toplacing the die 102 on the prepared substrate 104 for bonding. In animplementation, the dies 102 are activated while on the pick and/orplace tools. Such cleaning and activating steps performed while the die102 is on the tool may be in addition, or as an alternative to, certainof the processes described above, though certain details of the stepsdescribed above may be performed in a similar fashion.

As an example, the die 102 may be carried by the pick and place toolsthrough a wet cleaning station, an atmospheric plasma, a laserexplosion, or the like, on the way to the substrate 104. In someembodiments, the dies 102 are not rinsed after activation and prior tobonding. In other embodiments, the dies 102 may be rinsed with deionizedwater, a prepared formulary, or the like, at a process step prior tobonding. In some embodiments, the dies 102 are picked from the dicingtape and flipped by the pick tool to present the back surface 110 of thedie 102 to the bonding tool. The face of the pick tool may be in directcontact with the surface 106 to be bonded. The bonding tool head couplesto the back side 110 of the die 102, and brings the prepared surface 106of the die 102 to the prepared surface 108 of the substrate 104 forbonding.

In still other embodiments, the prepared surface 106 of known good dies102 are picked from the dicing tape and flipped by the pick tool topresent the back side 110 of the die 102 to the bonding tool. The faceof the pick tool may be in direct contact with the surface 106 to bebonded. The bonding tool head couples to the back side 110 of the die102 and brings the prepared face 106 of the die 102 (such as die 102B atFIG. 1C) to the exposed prepared surface 110 of known good dies 102(such as die 102A at FIG. 1C) stacked and bonded to the substrate 104for bonding.

For instance, after cleaning and activating the second surface 110 ofthe first die 102A and the first surface 108 of the substrate 104, theprepared first surface 106 of second known good dies 102B may besimilarly picked from the dicing layer and bonded to the prepared secondsurface 110 of the first die 102A. Multiple known good dies 102(A-N) maybe stacked over the substrate 104 or over one another, or combinationsthereof.

In another embodiment, the prepared dies 102 are picked from the dicingtape and flipped by the pick tool to present the back side 110 of thedie 102 to the bonding tool. The bonding tool head couples to the backside 110 of the die 102, and an electrostatic device may be used toremove particulates from the bonding surface 106 prior to attaching theprepared face 106 of the die 102 to the prepared surface 108 of thesubstrate 104 for bonding. Moreover, the pick and place tool may have aporous surface for contacting the bonding surface.

In other examples, carbon dioxide particles or compressed gas, forexample nitrogen, may be applied at a glancing angle to the preparedsurface 106 to remove spurious particulates from the prepared surface106 prior to bonding. In some applications hot compressed gas, forexample, hot nitrogen may be applied to remove spurious particles orexcess moisture from the prepared surfaces 106 just before the bondingoperation. The pressure of the compressed nitrogen may range between 20to 300 psi and preferably between 50 to 150 psi. Similarly, thetemperature of the compressed gas, for example nitrogen, may rangebetween 25 to 100° C. and preferably between 50 and 90° C. The fluidsurface cleaning time may range between 2 to 1000 milliseconds andpreferably under 200 milliseconds.

Activating One Bonding Surface

In most of the examples above, the prepared surface 108 of the substrate104 and the prepared surface 106 of the dies 102 are each plasma cleaned(ashing with oxygen plasma is the most common process) and activatedprior to bonding the dies 102 to the substrate 104. In certainembodiments, however, just one of these bonding surfaces (e.g., theprepared surface 108 of the substrate 104 or the prepared surface 106 ofthe dies 102) is plasma cleaned and/or activated prior to bonding thedies 102 to the substrate 104.

In the embodiments, eliminating surface ashing and activation process oneither the dies 102 or on the substrate 104 can decrease process relateddefects and improve yields, as well as reduce costs for forming thebonding surfaces. For instance, eliminating the process steps can resultin eliminating contamination of the bonding surface 106 by the dicingtape/plasma interaction product and reducing or eliminating metal lossfrom wiring layers. Additionally, damage to the dicing frame or gripring can be minimized so the frame or grip ring life can be extended.Further, throughput can be improved with fewer process steps taken, andwith no reduction in bond energy when activating one surface (106, 108)instead of both surfaces (106, 108). Total cost of ownership is reducedwith the elimination of plasma chambers for die 102 cleaning as well.

Cleaning and Preparing the Substrate—Alternate Embodiment

The illustrations of FIGS. 7A and 7B show example processes 700 and 750respectively, describing alternate processes for cleaning and preparingthe substrate 104 (process 700) and the dies 102 (process 750) forbonding. Again, although the illustrations (and the associateddescriptions) describe bonding dies 102 to a substrate 104, the processcan be used to bond wafers to wafers, dies to dies, dies to wafers, andso forth. Moreover, the substrate 104 described in these processes canalso refer to the back side of a die 102 already mounted to another die102 or to a larger substrate, such as a wafer.

Referring to FIG. 7A, a process 700 is shown for preparing the bondingsurface 108 of the substrate 104. At block 702, the substrate 104 isfabricated, and a bonding surface 108 is formed on the substrate 104 atblock 704, in like manner as described with regard to process 200 atFIG. 2. For example, the bonding surface 108 is prepared for directbonding without adhesive. In some embodiments as disclosed earlier,fabrication of the substrate 104 may also include forming the bondingsurface 108.

At block 706, the bonding surface 108 of the substrate 104 is cleaned,for example, with a wet process with solvent, or the like. Block 708follows, including a dry clean process with an oxygen plasma (commonlyreferred to as ashing) or other plasma chemistry, to remove anyinorganic and organic contaminants. The plasma may be provided throughan atmospheric, downstream, or reactive ion etching (RIE) process, forinstance. The substrate 104 may be re-cleaned, including scrubbed withdeionized or mildly alkaline water or other suitable solution to removeany particles from the ashing process. In some embodiments, the bondingsurface 108 of the substrate 104 may be rendered hydrophobic by cleaningmethods, before storing the substrate 104 for subsequent use or beforethe activation step. At block 710, the bonding surface 108 is activatedusing a nitrogen plasma process, for example, to prepare the substratesurface 108 for bonding. In various examples, the activation process isknown to improve bond strength. The substrate 104 may also be rinsedwith deionized water to remove potential contaminating particles fromthe activation process, if desired.

Cleaning and Preparing the Dies—Alternate Embodiment

Referring to FIG. 7B, a process 750 is shown for preparing the bondingsurface(s) 106 and/or 110 of the die(s) 102. At block 752, the dies 102are fabricated on a wafer, and at block 754 a bonding surface 106 isformed and prepared on a surface of the dies 102, as describedpreviously with regard to process 300 at FIG. 3. For example, thebonding surface 106 is prepared for direct bonding without adhesive. Atblock 756 one or more protective layers may be added to the bondingsurface 106, as also described above, and at block 758, the wafer issingulated into a plurality of dies 102.

At block 760, the process includes removing the protective layer(s) fromthe bonding surface 106. At block 762, the bonding surface(s) 106 arecleaned using a chemical formulary, as described further below. In somecases, the protective layer(s) may be cleaned from the dies 102 whilethe dies 102 are held on the dicing tape by the frame or grip ring,which also cleans debris resulting from the dicing process. When a UVrelease tape is used, UV exposure to reduce adhesion strength betweenthe die 102 and the tape to release the die 102 can be performed eitherbefore the die cleaning process, or after the die cleaning process, ifdesired.

Advantages to the process 750, including cleaning (or other processing)the dies 102 while on the frame or grip ring, or on a polymeric sheetinclude: elimination of polymeric residue from oxygen and nitrogenplasma steps; reduced processing steps and cycle times formanufacturing; the dies 102 may not need to be activated when thesubstrate 104 has been activated; the bond energy of the process(including without the die activated) is comparable to a like processwhere the dies 102 are activated along with the substrate 104activation; and higher throughput of manufactured devices.

Example Chemical Formulary

Referring to FIG. 7B, after the protective layer is cleaned from thesingulated dies 102 (and optionally after the dies 102 are cleaned witha plasma process) the dies 102 may be exposed to a chemical formularyfor a predetermined time to clean and prepare the die surface(s) 106and/or 110 for bonding. In the embodiment, the prepared formularycomprises glycerated diluted hydrofluoric acid or buffered hydrofluoricacid or ammonium fluoride, organic acid, and deionized water, with orwithout a stabilizing additive. In some embodiments, the formulary maycomprise an inorganic or organic acid containing a fluoride ion. It ispreferable that the content of the fluoride ion be less than 2% andpreferably less than 0.5% and, in some cases, preferably less than 0.1%.Examples of the sources of fluoride ions may include hydrofluoric acid,buffered oxide etch, ammonium fluoride, or tetrabutylammonium fluoride.

The formulary may also comprise aliphatic or non-aliphatic organicacids, and more than one organic acid may be used in the formulary. Theorganic acid content of the formulary may typically be less than 2% andpreferably less than 1%, and preferably less than 0.1%. Examples oforganic acid may include formic acid, acetic acid, methyl sulfonic acidand their likes. In some embodiments, mineral acids (for example, a verysmall amount of sulfuric acid) may be used. However, the amount usedshould not roughen the surface of the metallic layer at the bondingsurface 106.

In various embodiments, glycerol is incorporated into the formulary,where the content of glycerol may vary between 0.5 to 25% of theformulary, and preferably under 10%. In other applications, a very smallamount of amide, amines, butylated hydroxyanisole (BHA), butylatedhydroxytoulene, or organic carbonates may be added to the formulary. Inother embodiments, the formulary may be mildly alkaline with a pHpreferably less than 9.5 and preferably less than 8.5. It is preferablethat the total content of these additional additives be less than 5% andpreferably less than 1%.

It is also desirable that a complexing agent that suppresses the removalor etching or roughening of the surface of the metallic layer at thebonding surface 106 be incorporated into the formulary. In the case ofcopper, for example, a suitable copper complexing agent with one or moretriazole moieties may be used. The concentration of the complexing agentmay be less than 2%, and preferably less than 1%, 0.2%, and less than100 ppm and less than 5 ppm in some instances. After exposure to theformulary, the dies 102 are placed by the pick tool and bonded to thesubstrate 104, without activation (and optionally without ashing).

After cleaning with the formulary, the dies 102 may be rinsed (forexample with deionized water), and may be activated. A nitrogen-basedRIE process or downstream plasma method or other plasma cleaning stepsincluding atomic layer cleaning methods can be used to activate the diesurface 106 and/or clean any remaining residue or undesirable materialsfrom the surfaces of interest. As an option, the dies 102 may be rinsedwith deionized water after activation.

At the end of the die 102 forming and preparation process, the dies 102may be bonded to the prepared bonding surface 108 of the substrate 104(as illustrated in FIG. 1B). For a multiple-die stacking arrangement asshown in FIG. 1C, after placing each die 102A on the substrate 104 (oranother die 102N), the second bonding surface 110 (backside) of theplaced die 102A may be planarized, cleaned, and activated (as needed andas discussed above) before direct bonding a next die 102B to thebackside surface 110 of the previous die 102A. Any added die 102(A-N)may have a prepared bonding surface 106 and/or 110, which may or may notbe activated.

Forming a Hydrophobic Bonding Surface

In another embodiment, the surface of the dies 102 or substrate 104 orboth may be rendered hydrophobic by one or more of the cleaning stepsdescribed above. One advantage of rendering the surface (106, 108,and/or 110) itself hydrophobic is that hydrophobic surfaces may be lessprone to particulate contamination, and are easily cleaned withcompressed fluids such as nitrogen, carbon dioxide or carbon dioxideparticles. The bonding surface 106 or 110 of the die 102 (while in thedicing frame, for instance) or the bonding surface 108 of the substrate104 may be rendered hydrophobic by exposing their non-activated surfaces(106, 108, and/or 110) to the very dilute fluoride containingformularies described above, then rinsing and drying the exposed surface(106, 108, and/or 110). According to this technique, a cleaned,unactivated die 102 with a hydrophobic surface 106 and/or 110 may beattached to the surface 108 of a prepared, activated substrate 104 (orto another die 102) for bonding. Similarly, a cleaned, unactivatedsubstrate 104 with a hydrophobic surface 108 may be bonded to thesurface 106 of an activated die 102 or dies 102. Generally, nitrogenactivation of a surface (106, 108 and/or 110) tends to render thesurface (106, 108 and/or 110) hydrophilic. Annealing the mated surfaces(106, 108 and/or 110) improves the bond energy between the matedmaterials. Generally, the higher the bonding temperature, the higher theenergy need to separate the mated materials.

Additional Process Step Reductions

In a further embodiment, after the protective layer is cleaned from thesingulated dies 102, the dies 102 are placed by the pick and place toolsand bonded to the substrate 104, without exposure to the formulary,ashing, or activation. In various embodiments, with the substrate 104activated and the die 102 not activated, the bond energy (or thestrength of the bond) of the resulting direct bond is sufficient for theDBI formation process. For instance, in some embodiments, the bondenergy is approximately 1000 mJ/m², which meets minimum bond energyrequirements for a suitable direct bond according to somespecifications. Further, elimination of certain oxygen and nitrogen RIEsteps eliminates the dicing tape/plasma reaction by-products which candeposit on the die 102 surface, as well as the cost and time ofadditional process steps.

Annealing

As a final step in the bonding process for all of the embodimentsdiscussed, the die 102 and substrate 104 may be heated above ambienttemperature (annealed) to form the metal-to-metal joint. In theseembodiments, the high temperature of the annealing process causes themetal (e.g., Cu) in the bonding layer (106, 108 and/or 110) to expandmore than the dielectric material (e.g. oxide) surrounding the metal.The deferential in CTE allows the metallic layer(s), which may berecessed below the bonding surface (106, 108 and/or 110) at roomtemperature, to expand to bridge the gap between the two mating surfacesof the dielectric materials and form electrically conductive jointsduring the anneal.

In one embodiment, a multi-die stack is not annealed after each die 102is placed, but instead the whole stack is annealed once all of the dies102 of the stack are placed. Alternatively, a low temperature anneal maybe used after each die 102 is placed. Further, the back surface 110 ofthe bonded die 102 is cleaned and prepared to accept an additionalcleaned die 102 or die stack. The substrate 104 with a multi-die stackmay be thermally annealed at a higher temperature for the opposing metalfeatures at the various bonding interfaces to mate intimately.

In some embodiments, after disclosed processes, the substrate 104 may besingulated by known methods to form a new structure comprising of dies102, directly bonded to a smaller singulated substrate 104 (not shown).In one embodiment, a bonding surface area of a die 102 is smaller thanthe bonding surface of the singulated substrate 104.

Pick and Place Tool Preparation

In various implementations, the pick and place tools are designed ortreated to minimize the opportunities to contaminate the dies 102. Thepick and place tools (or portions of them) may be formed from a material(or coated with a material) selected for desired hydrophobic properties.For instance, the tools may be coated with a material such aspolytetrafluoroethylene (PTFE), or another hydrophobic material.

Also, the tools may be prepared (e.g., with a structural design, with apredetermined treatment, etc.) to be resistant to contaminants or toavoid passing contaminants to the dies 102. As such, the face pickprocess (e.g., picking the dies 102 at the prepared bonding surface 106and/or 110) does not degrade bonding quality while picking from the diesurface 106 and/or 110 to be bonded. Alternatively, the surface 106and/or 110 of the die 102 can be chemically treated to have hydrophobicproperties to minimize particles sticking to the surface 106 and/or 110from the pick tool. Also, at desirable intervals, the surface of thepick and place tool can be cleaned to remove potential sources of diesurface contaminants. One or more surfaces of the tool may be porous.

The techniques, components, and devices described herein are not limitedto the illustrations of FIGS. 1A-7B, and may be applied to otherdesigns, types, arrangements, and constructions including with otherelectrical components without departing from the scope of thedisclosure. In some cases, additional or alternative components,techniques, sequences, or processes may be used to implement thetechniques described herein. Further, the components and/or techniquesmay be arranged and/or combined in various combinations, while resultingin similar or approximately identical results.

CONCLUSION

Although the implementations of the disclosure have been described inlanguage specific to structural features and/or methodological acts, itis to be understood that the implementations are not necessarily limitedto the specific features or acts described. Rather, the specificfeatures and acts are disclosed as representative forms of implementingexample devices and techniques.

1. A method of forming a microelectronic assembly, comprising: preparinga bonding surface of a first substrate, including: planarizing thebonding surface of the first substrate to have a first predeterminedminimal surface variance; and plasma activating the bonding surface ofthe first substrate; preparing a bonding surface of a second substrate,including planarizing the bonding surface of the second substrate tohave a second predetermined minimal surface variance; mounting thesecond substrate to a dicing sheet or dicing tape held by a dicingframe; singulating the second substrate into a plurality of dies whilethe second substrate is mounted to the dicing sheet or dicing tape, eachdie of the plurality of dies having a bonding surface comprising aportion of the bonding surface of the second substrate; processing theplurality of dies while the plurality of dies is mounted to the dicingsheet or dicing tape; selecting a die of the plurality of dies using apick and place tool; and directly bonding the bonding surface of the dieto the bonding surface of the first substrate without adhesive, andwithout activating the bonding surface of the die.
 2. The method ofclaim 1, further comprising: applying a protective coating to thebonding surface of the second substrate; and cleaning the protectivecoating from the plurality of dies while the plurality of dies ismounted to the dicing sheet or dicing tape.
 3. The method of claim 2,further comprising cleaning one or more surfaces of the plurality ofdies or the first substrate using a chemical reagent, a megasonictransducer, and/or a mechanical brush.
 4. The method of claim 1, furthercomprising processing the die of the plurality of dies while the die isheld by the pick and place tool, the processing including insitucleaning the bonding surface of the die.
 5. The method of claim 1,wherein the dicing sheet or dicing tape is a first dicing sheet or afirst dicing tape, and further comprising transferring the plurality ofdies to a second dicing sheet or second dicing tape to avoidcontamination to the plurality of dies from particles or residue on thefirst dicing sheet or the first dicing tape and/or to enable use ofsolvents or chemicals that are not compatible with the first dicingtape.
 6. The method of claim 1, further comprising minimizing exposureof the dicing sheet or dicing tape to contaminants by shielding exposedportions of the dicing sheet or dicing tape beyond the plurality ofdies, minimizing exposed portions of the dicing sheet or dicing tapebetween dies of the plurality of dies, and/or shortening one or moreprocess step durations.
 7. The method of claim 1, further comprisingselecting the die of the plurality of dies at the bonding surface of thedie using the pick and place tool, without contaminating the bondingsurface of the die to an extent greater than a predefined bondingquality specification.
 8. The method of claim 7, wherein a surface ofthe pick and place tool is in direct contact with the bonding surface ofdie when the die is selected with the pick and place tool.
 9. The methodof claim 7, wherein a surface of the pick and place tool is porous. 10.The method of claim 7, wherein a surface of the pick and place tool isformed from a hydrophobic material or is coated with a hydrophobicmaterial.
 11. The method of claim 1, further comprising treating thebonding surface of the die to become hydrophobic.
 12. The method ofclaim 1, wherein the microelectronic assembly comprises a stack ofmultiple directly bonded dies of the plurality of dies, and wherein themicroelectronic assembly is low-temperature annealed after placement ofeach die on the stack.
 13. The method of claim 1, wherein themicroelectronic assembly comprises a stack of multiple directly bondeddies of the plurality of dies, and wherein the microelectronic assemblyis annealed only after placement of a plurality of dies on the stack.14. The method of claim 1, wherein the bonding surface of the die of theplurality of dies is a first bonding surface and wherein the die of theplurality of dies includes a second bonding surface opposite the firstbonding surface of the die, the method further comprising: preparing thesecond bonding surface of the die, including: planarizing and/orcleaning the second bonding surface of the die to have a thirdpredetermined minimal surface variance; and plasma activating the secondbonding surface of the die; preparing a first bonding surface of anadditional die, the first bonding surface of the additional die having afourth predetermined minimal surface variance; and directly bonding thefirst bonding surface of the additional die to the second bondingsurface of the die without adhesive, and without activating the firstbonding surface of the additional die.
 15. A method of forming amicroelectronic assembly, comprising: preparing a bonding surface of afirst substrate; preparing a bonding surface of a second substrate,including planarizing the bonding surface of the second substrate tohave a predetermined minimal surface variance; mounting the secondsubstrate to a dicing tape; singulating the second substrate into aplurality of dies while the second substrate is mounted to the dicingtape, each die of the plurality of dies having a bonding surfacecomprised of a portion of the bonding surface of the second substrate;processing the plurality of dies while the plurality of dies is mountedto the dicing tape; selecting a die of the plurality of dies using apick and place tool; and directly bonding the bonding surface of the dieto the bonding surface of the first substrate without adhesive, andwithout activating the bonding surface of the die.
 16. The method ofclaim 15, further comprising planarizing the bonding surface of thefirst substrate to have another predetermined minimal surface variance;and plasma activating the bonding surface of the first substrate. 17.The method of claim 15, further comprising planarizing the bondingsurface of the first substrate to have another predetermined minimalsurface variance; and plasma asking and plasma activating the bondingsurface of the first substrate.
 18. The method of claim 15, furthercomprising rendering the bonding surface of the die hydrophobic prior todirectly bonding the bonding surface of the die to the bonding surfaceof the first substrate.
 19. A method of forming a microelectronicassembly, comprising: preparing a bonding surface of a first substrate,including: planarizing the bonding surface of the first substrate tohave a first predetermined minimal surface variance; and plasmaactivating the bonding surface of the first substrate; preparing abonding surface of a second substrate, including planarizing the bondingsurface of the second substrate to have a second predetermined minimalsurface variance; mounting the second substrate to a dicing sheet ordicing tape held by a grip ring or dicing frame; singulating the secondsubstrate into a plurality of dies while the second substrate is mountedto the dicing sheet or dicing tape, each die of the plurality of dieshaving a bonding surface comprised of a portion of the bonding surfaceof the second substrate; processing the plurality of dies while theplurality of dies is mounted to the dicing sheet or dicing tape;selecting a die of the plurality of dies using a pick and place tool;and directly bonding the bonding surface of the die to the bondingsurface of the first substrate without adhesive.
 20. The method of claim19, further comprising rendering the bonding surface of the diehydrophobic prior to directly bonding the bonding surface of the die tothe bonding surface of the first substrate.
 21. The method of claim 19,further comprising activating the bonding surface of the die.
 22. Themethod of claim 19 further comprising applying a megasonic cleaner toclean the plurality of dies or the first substrate or the plurality ofdies and the first substrate. 23.-41. (canceled)